The technology disclosed herein generally relates to methods for polishing end faces of optical fibers and, more particularly, relates to polishing end faces of plastic optical fibers.
An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis. The fiber consists of a transparent core surrounded by a transparent cladding layer (hereinafter “cladding”), both of which are made of dielectric materials. Light is kept in the core by the phenomenon of total internal reflection. To confine the optical signal in the core, the refractive index of the core is greater than that of the cladding. The boundary between the core and cladding may either be abrupt, as in step-index fiber, or gradual, as in graded-index fiber.
An important aspect of a fiber optic communication is that of connecting two fiber optic cables such that optical loss is minimized. Temporary or semi-permanent connections are made by means of specialized optical fiber connectors. It is often necessary to align an optical fiber with another optical fiber. This can involve either carefully aligning the fiber and placing it in contact with the device, or using a lens to allow coupling. In some cases the end of the fiber is polished into a curved form that makes it act as a lens.
A plastic optical fiber (POF) is an optical fiber that is made of polymeric materials. Similar to glass optical fiber, POF transmits light (for illumination or data) through the core of the fiber. Its chief advantage over the glass product, other aspects being equal, is its robustness under bending and stretching. Typically, the core is made of poly(methyl methacrylate) (PMMA) or polystyrene (96% of the cross section in a fiber 1 mm in diameter), while the cladding is made of silicone resin. The former have refractive indices of 1.49 and 1.59 respectively, while the latter has a refractive index ˜1.46.
PMMA is a soft material and easily damaged using polishing processes typically used to polish glass optical fibers. In addition, POF has a typical diameter of 1 mm whereas glass optical fiber typically has a diameter of 0.125 mm. This difference in diameter makes it more difficult to achieve uniformity with a high-quality end face when polishing POF.
An existing solution uses a standard glass optical fiber polishing process to polish POF end faces. However, glass polishing processes produce POF end faces which are full of scratch marks, pitting and cracks. These defects give the POF connector a very high loss and render such connectors unsuitable for use in avionics POF networks and systems onboard commercial aircraft. An automatic POF polishing machine is commercially available that uses a diamond-based polisher, but this machine does not produce high-quality POF end faces due to the roughness of the diamond grit. It is useful for short-distance POF optical systems where the loss produced by the connector is not crucial for the system operation. An example of this type of application is limited to automobiles where the total POF link is 10 meters or less.
Plastic optical fiber can be substituted for copper conductors in an aircraft data bus network. For commercial avionics applications, a defect-free POF end face finish is important because the POF link length in commercial aircraft ranges from 30 to 100 meters. Exacerbating the need of low-loss POF end faces is the high operating temperature environment of commercial aircraft where the optical power budget provided by the transmitter and receiver is reduced at high operating temperature.
In some aircraft models, the avionics system comprises one or more POF converters in the forward section of the aircraft and one POF converter in the aft section of the aircraft. Each forward POF converter is connected to the aft POF converter by POF having a relatively long length. The high optical loss of the POF introduces an optical loss in the connections between the forward and aft POF converters. In addition, there are multiple POF optical connectors in the POF link. The connector loss depends on the POF end face quality. A poor POF end face can introduce an additional optical loss per connector. The provision of smooth POF end faces is important to reduce the connectors' optical coupling loss for avionics POF networks where the optical link budget is very tight due to a relatively long POF length.
There is a need for a polishing process that can produce defect-free POF end faces for use in POF networks having long POF links, such as in avionics POF networks.